For me, most of the secret is in buffing spells. Sarevok is a monster with his Deathbringer Assault and high-level fighter powers. With the right gear and buffs, he slaughters everything. He's also a perfect candidate for dual-classing, since his stats are awesome. You can play Sarevok as a pure fighter and do well with him...or, you can dual him and have great fun. Also, I admit that when I click on an enemy for him to attack, I get a kick out his homicidal laugh.
Keldorn is much more limited by way of options than Sarevok in your party, but the options he has available makes him just as valuable in my opinion. Keldorn can make it happen when you really need it, so taking care of him and buffing him up is the key. As it happens, this is the key for all the NPCs in the game...even Anomen.